This invention relates to mobile radiotelephone systems and particularly to such systems which employ a tone for supervision of call status.
Cellular radio communication systems are being increasingly considered as arrangements which can allow substantially higher numbers of mobile radiotelephone subscribers to have access to a relatively limited number of radio communication channels. One form of such a cellular system is described in a report entitled High Capacity Mobile Telephone System Technical Report, December 1971, prepared by Bell Telephone Laboratories, Incorporated, and filed with the Federal Communications Commission in that month under Docket 18262. It has been proposed in such a cellular radio system to require each base station, sometimes called a cell site, which communicates on a set of channels in common with other base stations that are within a potential cochannel interference radius, to combine with its access, or call setup, channel signals a supervisory tone which is unique to that base station among the group of possibly interfering base stations. Such a tone is transponded by a mobile unit receiving that channel. Any base station receiving the transponded tone employs it to determine whether or not the transponding mobile station has been captured by an interfering base station and whether or not the receiving base station has captured an interferring mobile station. In this context the term "captured" is utilized with reference to the well-known phenomenon of frequency modulation capture. Three examples of cellular mobile radiotelephone systems are found in the U.S. Pat. Nos. 3,663,762 of A. E. Joel, 3,898,380 of G. D. Wells et al., and 3,906,166 of M. Cooper et al.
It is desirable in the cellular type of system to provide some indication of the location of a mobile unit engaged in a call connection in the system. The patentees in the three aforementioned patents all employ some form of signal level determination as an indication of mobile unit position so that a decision can be made as to when to handoff a mobile unit between adjacent cell base stations. However, the aforementioned supervisory tone is also useful for propagation delay ranging as indicated for example at page 3-17 in the aforementioned technical report. Another propagation delay system is indicated at page 4-64 of "An Application for a Developmental Cellular Mobile and Portable Radiotelephone System in the Washington-Baltimore Northern-Virginia Area". This application was submitted to the Federal Communications Commission by the American Radio Telephone Service, Incorporated and dated Feb. 14, 1977.
Propagation delay ranging, or phase ranging as it is sometimes called, is accomplished in the mentioned technical report by utilizing one of plural system supervisory tones near the upper end of the voice band and which are usually very close together in the frequency spectrum, e.g., within about .+-.0.5% of one another. Any use of the supervisory audible tone, including its use for ranging, requires a receiving station to determine that it is receiving the true tone for its channel. Detection of a certain one of several such closely spaced tones by conventional filtering techniques usually requires complex and/or difficult-to-integrate circuits. Another technique that has been proposed, for example, in the E. J. Addeo, U.S. Pat. No. 4,025,853, depends upon product modulating a transponded version of the tone, after selection by a phase-locked loop, with a transmitted version of the tone. Then the modulator output is bandpass filtered in a band including the possible difference frequencies among the system supervisory tones. If no such difference frequency is detected, it is presumed that the correct supervisory tone is being received. If such a difference frequency is detected, it is presumed that the wrong supervisory tone is being received, i.e., that interference is present from another usage of the same channel in the system, and a call in progress is terminated. It has been found that reflections from topological features can cause multipath sidebands in the difference frequency band at the modulator output and of sufficient amplitude to cause call termination even when the correct supervisory tone is present.